In a wok, heat the oil and stir-fry the ginger, garlic and Chinese five-spice powder for about 1 minute, until fragrant. Add the mushrooms and sauté for another 3 minutes. Add the tofu, and sauté with the mushrooms for another 5 minutes. Add the bok choy, soy sauce and scallions, and cook for another 2 minutes.

Mix the cornstarch with the vegetable stock, and add to the wok. Simmer, stirring constantly, until the sauce is well thickened.